1. Field of the Invention
The present invention refers to a method of controlling a direct-current drive, in particular a direct-current door drive.
2. Description of Related Art
The safety requirements which are to be met by the control of direct-current drives, used e.g. for opening and closing doors, garage doors, electric sliding roofs and the like, are very high. These safety requirements include the recognition of dangerous situations. If there is an obstacle in the path of movement of the element to be actuated, e.g. a garage door, switching off or reversing of the door must be initiated immediately. To avoid damage and injuries, the external forces acting on the door edge in the case of such switching off or reversing must not exceed specific limit values. Such limit values are prescribed by the legislator. In addition, it is endeavoured to keep the forces actually occurring as low as possible by special methods.
In the methods for controlling direct-current drives known from the prior art, the motor current applied to the direct-current drive is compared with an absolute limit value or with detected reference values.
In a first known method for controlling a direct-current drive, the motor current is compared with an absolute, predetermined, constant limit value. As soon as the instantaneous motor current exceeds a predetermined absolute value, the direct-current drive will be switched off. The disadvantage of this first known method is that the maximum driving power is strongly limited. A further problem entailed by this known method arises when the force required by a door depends strongly on the door position. In this case, positions exist at which the force at the door edge may exceed the admissible values without switching off of the direct-current drive being caused.
In order to avoid the problems described hereinbefore, a second method, which is known in the prior art, was developed; this method is known from DE 4214998 A1 and includes the step of comparing the motor current, which is applied to the direct-current drive, with a current determined by a learning trip in dependence upon the door position. In this method, which is based on reference measurements, the necessary motor current or the target speed is measured for various positions during an undisturbed trip of the door and stored as a reference value. Inadmissible deviations from the stored reference values will then result in switching off during subsequent trips. This known method is, however, problematic insofar as in cases in which a door is stopped between the end points of a trip (the point at which the door is closed or the point at which the door is open) and then started again, a high starting current will occur when the door is being re-started. For this high starting current at a position between the two end points, reference values do not exist. In order to avoid switching off or reversal of the door in this case, higher limit values for an admissible operation of the door must be provided in dependence upon the respective case of use when this method is employed. Another possibility of preventing the limit values from being exceeded is to disable the supervision during the starting period.
The above-mentioned problem arising when a door is being restarted after a stop at a position between the two end positions is clearly shown on the basis of a graph represented in FIG. 5. As can be seen from FIG. 5, the necessary motor current is there plotted against the position of a door. More precisely, the necessary motor current is plotted against the range between the door being opened and closed. Curve a) represents the motor current during a so-called normal trip. For this normal trip reference values were ascertained during the learning trip to which a predetermined tolerance value was added, whereby the limit value curve b) is obtained, which is shown in FIG. 5. Curve c) shows the problem which arises when the door was stopped at an arbitrary position between the two end positions and when it has to be re-started subsequently. In this case, the necessary motor current increases abruptly so that it would clearly exceed the limit value curve b). In order to avoid this problem, the limit value curve must be chosen such that, even if the door is re-started after having been stopped between the end positions, switching off of the direct-current drive by the high motor current occurring during re-starting can reliably be avoided. This, however, entails the problem that very high limit value curves b) must be chosen so that, in cases in which the door meets with an obstacle during its trip, switching off or reversing of the door will only be initiated after a comparatively long time. In one case, this may have the effect that the direct-current drive is damaged due to an excessive load applied thereon. In another, much more critical case, in which the above-mentioned obstacle is a human being, the switching off, which has been effected too late due to the higher limit values, may cause substantial injuries of the human body.